A device of the above described kind is previously known from U.S. Pat. No. 1,285,079, which device constitutes a part of a teat liner which is usually made of nitrile rubber.
During manufacture of a rubber product, there always arise, in the material, discontinuities, which may consist of e.g. air or foreign particles. When load is put on the rubber product cracks may arise, which in that case firstly occurs in connection with the discontinuities.
It is commonly known that surface cracks are more dangerous for the strength of the rubber product than cracks inside the rubber product. It is accordingly especially important to hamper growth of cracks in the surface layer of the rubber product.
When a rubber product ages micro cracks also arise in its surface layer because of reduced elasticity of the rubber. These cracks propagate i.e. get larger, especially during loading of the rubber product. One reason for reduced elasticity of rubber in course of time is that this material is hygroscopic.
A newly produced teat liner made of nitrile rubber contains less than 0,3% water. A teat liner is during use contacted by milk during milking and detergent solution during subsequent cleaning. The milk as well as the detergent solution contains water, which is absorbed by the rubber material of the teat liner. The more water that is absorbed by the rubber material the more the teat liner loses its elasticity. Furthermore the tension of the rubber material is increased owing to this. When a known teat liner made of nitrile rubber is worn out, it contains about 3% water.
At this stage surface cracks have already arisen in the teat liner because of the reduced elasticity in combination with the movement that the teat liner has been forced to perform during milking. The movement has caused tensions in the teat liner that have promoted the growth of the cracks.